1. Field of the Invention
This invention relates to an information recording medium, and more particularly to an information recording medium for writing and/or reading information by means of laser beam of high energy density.
2. Description of Prior Arts
Information recording mediums utilizing a beam of high energy density such as laser beam have been developed in recent years and put to practical use. Such a recording medium is called an optical disc and the practical applications thereof have been found, for example, as video disc and audio disc as well as disc memory for large-capacity computer and large-capacity static image file.
The optical disc basically comprises a disc-shaped transparent substrate of a plastic or glass material and a recording layer composed of a metal such as Bi, Sn, In or Te or a semi-metal provided on said substrate. Writing of information on the optical disc can be conducted, for example, by irradiating the optical disc with a laser beam. Under irradiation with the laser beam, the irradiated area of the recording layer of the optical disc absorbs the energy of beam and a rise in temperature locally occurs. As a result, a chemical or physical change takes place to alter the optical characteristics of the recording layer in the irradiated area, whereby the information is recorded. Reading of information from the optical disc is conducted by irradiating the optical disc with a laser beam. The information can be reproduced by detecting reflected light or transmitted light corresponding to the alteration in the optical characteristics of the recording layer.
As a disc structure for protecting the recording layer, there has been recently proposed an air-sandwich structure in which a recording layer is provided on at least one of two disc-shaped substrates and positioned inside the two substrates opposed to each other at a given distance by interposing a ring-shaped inner spacer and a ring-shaped outer spacer between the substrates so as to form a closed space surrounded by these two substrates and these two spacers. In the optical disc having such a structure, the recording layer is not directly exposed to the atmosphere and the recording and reproduction of information is conducted with a laser beam passing through the substrate. Accordingly, in general there is no possibility that the recording layer is physically or chemically damaged or the surface thereof is polluted with dust which is to interfere with the recording and reproduction of information.
Previously, as described in Japanese Patent Provisional Publications No. 50(1975)-126237 and No. 57(1982)-205193, it is known that a thermal barrier layer can be provided to the optical disc to enhance the recording sensitivity. The thermal barrier layer is provided between the substrate and the recording layer and known to be made of an organic polymer such as polyester, polyethylene, polystyrene or acrylic polymer or made of a non-metallic compound such as silicon dioxide. This thermal barrier layer reduces loss of thermal energy given by the irradiation of a laser beam, the thermal loss taking place from the recording layer into the substrate layer through thermal conduction.
Further, it is known that a glass substrate such as a tempered glass substrate is employed as the substrate of the optical disc. The optical disc using a glass substrate is advantageous in that it rotates more stably, the surface thereof is harder, and the dimensional stability is higher, as compared with the optical disc using a plastic substrate such as a substrate of an acrylic resin.
In the course of study on the optical disc, the present inventors have noted that alkali metal ions such as Na.sup.+ and K.sup.+ and alkaline earth metal ions such as Mg.sup.2+ and Ca.sup.2+ (both are hereinafter represented by a term "alkali ions") contained in the glass substrate is apt to diffuse out on the surface of the substrate under influence of circumferential conditions such as temperature and humidity. The diffusing alkali ions likely give adverse effects on the recording layer provided on the substrate. In more detail, the alkali ions cause alteration of the characteristics of the recording layer as the diffusion progresses. Such alteration results in a decrease of the recording sensitivity, decrease of optical density utilized for reading the recorded information, decrease of bonding between the substrate and the recording layer, and/or deposition of precipitate between the substrate and the recording layer. These sometimes trigger abnormal functions of the recording medium.
The assignee of the present application has already filed a patent application (Japanese Patent Application No. 58(1983)-173676) disclosing an invention in which the glass substrate is undercoated with a polymer having a hydrophilic group to prevent the unfavorable effect of the alkali ions diffusing from the substrate. In more detail, this invention comprises providing between the substrate and the recording layer and undercoating layer of a polymer having a hydrophilic group such as carboxylic group to keep the recording layer from suffering from adverse effect of alkali ions liberated onto the surface of the substrate.
The material utilized for the aforementioned thermal barrier layer shows little ability to prevent the diffusion of the alkali ions. Thus, the constitution of the information recording medium of prior art comprising a glass substrate, a thermal barrier layer and a recording layer in order is not effective to keep the recording layer from the adverse effect of alkali ions. Therefore, the recording medium of prior art is liable to decrease in the recording sensitivity and to deteriorate in the quality of recorded information in the course of storage thereof. Further, an intermediate layer such as a metallic reflecting layer disclosed in the aforementioned Japanese Patent Provisional Publication No. 57(1982)-205163 is not effective to prevent the diffusion of alkali ions.
As described hereinbefore, the information recording medium is of a great value in a variety of arts. Accordingly, it is desired that the recording sensitivity is as high as possible, that the recording layer is kept stable as long as possible, and that error in reading the recorded information is as little as possible.